Translator

09 December 2012

BOBGRAM issued 9 Dec

WEATHERGRAM
YOTREPS
Issued 9 Dec 2012
Bob McDavitt's ideas for sailing around the South Pacific.
Disclaimer: Weather is a mix of pattern and chaos; these ideas are from the patterned world of weather maps, so please fine-tune to your place. Dates are in UTC unless otherwise stated.
The Atmosphere: The Southern Oscillation Index or SOI (30 day running mean) had a value of MINUS 1.01 back on 25 Aug. It has been gradually rising ever since and on 02 Dec was plus 0.4, and by 9 Dec it has relaxed to plus 0.25. Computer models are picking a neutral SOI for the cyclone season.

Tropical cyclones: Last week TC BOPHA left behind a death toll of over 500 in the Philippines. It has changed course and is heading to Philippines again as a tropical storm.

South Pacific Convergence Zone SPCZ
If you are in the tropics then this is a week when the best thing to do is to stay put.

The South Pacific Convergence Zone SPCZ is dividing into three different zones of activity—one over northern Coral Sea, one between Fiji and Samoa and a third over the Southern cooks. There is now a zone of light north to NW winds on the northern side of the SPCZ, to North of 5S. So the SPCZ is turning into a trough of falling pressure.
The zone over Southern Cooks is expected to develop into a tropical Low on Monday or Tuesday that should travel westwards. There is already some cyclonic curvature showing in the cloud imagery.
The zone between Samoa and Fiji is expected to develop into a Low next-by around Wednesday or Thursday and may well deepen quickly into a tropical Cyclone that may be slow-moving over Fiji during the 15-16 Dec weekend. Avoid.

SUBTROPICAL RIDGE STR
The High over Northern Tasman Sea/North Island tonight is expected to weaken over the next few days. A new High is moving east across Tasmania on Monday, tugging the STR southwards as is expected during summer. This new high is expected to slide across central NZ on Thursday and then out to the east of the North Island next week. Its departure is likely to be a factor in the track of the tropical low over Fiji to the south.

NZ/Tasman Sea
Last Thursday's trough had a wedge of cooler air pushing into the warm pool on its eastern In the western flank of the warm pool there were some NW winds – these had started as part of that heat wave over Australia in the previous week. As these NW winds encountered the west coast of Auckland the friction difference between land and sea causes a convergence zone along the west coast line. As the extra upward motion of the wedge reached this convergence zone, it was able to fire a line of thunderstorms and one of these was severe enough to trigger a tornado. A rare event for west Auckland and this one led to three fatalities, put 7 in hospital and damaged around 200 houses.

This week's troughs are not expected to bring much--- one should fade over the South island on Monday and the next should linger over southern NZ from Friday 14 to Sunday 16 Dec.

There is likely to be a build-up of heat showers over the inland North Island on Wednesday and Thursday 12/13 Dec.